The invention concerns a process for the separation of liquids from suspensions, in which suspension is fed on one side of at least one filter layer, and liquid separated from the suspension is carried away on the other side of this filter layer, and in which the amount of solids accumulating on the feed side of the filter layer through this separation of liquid, is mixed, through a relative movement between the filter layer and the suspension fed, with the latter, and at least partly carried away. The invention concerns, further, an apparatus for the separation of liquids from suspensions, with at least one filter layer, on the feed side filter surface of which is arranged a suspension chamber connected with a suspension feed and suspension and solids removal means, and on the removal side surface of which is arranged a filtrate chamber which receives the liquid passing through the filter layer, while in the suspension chamber, adjacent the filter surface, at least one drive device movable in relation to the filter surface, is provided.
A process and an apparatus of the said kind are known from German Disclosure No. 2,251,171. This filter apparatus is built up with a plurality of disc-form, coaxially arranged filter elements, of which the surfaces are provided with filter layers. Between two neighboring elements or filter surfaces, as the case may be, there is formed, in each case, a flat cylindrical section of the suspension chamber connected with a suspension feed, while in each of these suspension chamber sections there is arranged for the production of a flow of suspension, a drive device, also disc-form, a so-called disc agitator. The disc-form drive devices of all the suspension sections are also arranged coaxially on a shaft and are set in rotation by the latter. The disc-form drive devices divide the suspension chamber section, in each case, into two chambers, which are bounded, on the one hand, by the filter surface and on the other by the rotating discs of the drive device. Suspension flows axially, as a whole, through the suspension chamber, this movement taking place, within the individual suspension space chambers, substantially radially, outward and inward. Axial passages are situated only, on the one hand, at the circumference of the drive shaft; that is, in the central zone of the disc-form filter elements, and on the other hand, on the outer circumference of the disc-form drive devices. The flow of suspension, directed, respectively, inward and outward, may be set in turbulence by stays (bars, strips) arranged axially or inclined or in the manner of turbine blades, on the discs, and directed against the filter surfaces.
It has proved that in these known filter devices and their process of operation, with only turbulent flow of suspension, which may have additionally a component directed constantly against the filter surfaces, sufficiently high shear tensions within the flowing suspension in the zone next to the filter surface are difficult to obtain in such degree as would be necessary for keeping the filter surfaces free, in the long run, of accumulating solids, and thus for uninterrupted operation for long periods with high efficiency. Rather, in the known apparatus, cleaning and maintenance work would have to be carried out at relatively short intervals, which lead to a relatively low utilization of the machine, and because of the necessary dismantling of the filter device, associated with high operation cost.
The object of the invention, therefore, is to provide a filtration process and a corresponding apparatus, which is distinguished by improved clearing of the filter surfaces under the action of the suspension flow, and thus by a relatively high degree of utilization and low maintenance cost. The accomplishment of the object in accordance with the invention is defined by the claims.
An important feature of the process according to the invention, therefore, is an oscillating flow movement in the direction normal to the filter surface, by which, according to experience, an intensive mixing of the solids settling during the separation of liquid, with the flowing suspension, and thus a cleaning or keeping free of the active filter surface, is attained. It has proved especially effective, to produce a flow movement of the said kind, within the suspension, in a suspension chamber situated between two filter surfaces opposite each other, with an oscillating movement component between the filter surfaces. Through the alternate arrival and turning of the suspension mass against the filter surface, the movement energy of the flow is used especially effectively for freeing the filter surfaces. With this, there is given, preferably, a progressive flow movement of the suspension through the filter surface, in waves, in relation to the movement components oscillating in the direction normal to the filter, so that the whole expanse of the generally circular filter surface is uniformly included. Additionally, intensified eddy components may be produced within the suspension flow, which is important, above all, for the avoidance of solids precipitation in the zone of the thickened suspension.
The apparatus in accordance with the invention is distinguished by a turning of the suspension flow in the sense of an oscillating normal movement toward the filter surface, and namely, with the aid of surface elements arranged in succession in the direction of movement which have different distances and/or setting angles in relation to the filter surface. In this way, a dynamic pumping effect with strong normal components toward the filter surface is produced, but with an alternating opposite direction of this pumping effect in the successive surface elements of the drive device. Hence, the alternate suction and pressure effect on the solids settling in the zone of the filter surface leads to an especially effective loosening, detaching and carrying away of the solids, that is, to a highly effective clearing of the filter surfaces. This forced turning, which takes place with the aid of suitably arranged surface elements, is combined to particular advantage with the above-mentioned arrangement of passages between the surface elements. In this way there is given greater amplitude of oscillation and an intensified formation of eddies within the suspension flow.